This invention relates to a firing jig for an electronic element, which has a long life and allows production with a low cost.
The surface of a firing jig for an electronic element used in firing functional ceramics has been coated with a dense and less reactive coating material for preventing deterioration in properties of a material to be fired due to reaction of a component in the firing jig with the material or absorption of a component in the material in the firing jig.
A frequently used substrate for a firing jig is based on Al2O3xe2x80x94SiO2. The material is inexpensive and also highly resistant to thermal shock to allows a firing jig to have a longer life, so that it has been widely used as a substrate of a firing jig for an electronic element.
However, although the outer surface of the jig is coated with a dense coating film, repeated firing may cause penetration of SiO2 component in the substrate through the film structure and may remain in the surface of the jig. Thus, the SiO2 component may contaminate a material to be fired and then may adversely affect the properties of an electronic element.
In an alumina-based firing jig, as an Al2O3 content increases, a coefficient of thermal expansion increases, so that the jig becomes less resistant to thermal shock, leading to a shorter life.
Japanese Patent Laid-Open No. 10-158081 has disclosed that a zirconia flame coating film as a coating film may be formed to a thickness of 300 xcexcm on the surface of an Al2O3xe2x80x94SiO2-based substrate with an Al2O3 content of 65% or higher to improve durability; specifically, detachment of the flame coating layer in the jig may be reduced even after repeated cycles of heating and cooling as a firing jig. Such a configuration may prevent detachment of the flame coating layer, but properties of the electronic element placed and fired on the firing jig may be unstable, resulting in little or no increase in a jig life.
In view of the problems in the prior art described above, an objective of this invention is to provide a reliable firing jig for an electronic element by forming two or more spray coating layers on the surface of the substrate in the firing jig and defining some factors such as the contents of required materials for the substrate and individual spray coating layers for improving durability of the firing jig and stabilizing performance of the electronic element placed and fired on the firing jig.
This invention provides a firing jig for an electronic element comprising a substrate made of a ceramic and a spray coating layer made of a material less reactive to a material to be fired on the substrate, wherein two or more spray coating layers are formed on the surface of the substrate, and at least a spray coating layer adjacent to the substrate and a spray coating layer adjacent to the material to be fired are made of different materials.
In a firing jig for an electronic element according to this invention, it is preferable that the substrate is mainly based on Al2O3xe2x80x94SiO2, Al2O3xe2x80x94SiO2xe2x80x94MgO or SiC; the spray coating layer adjacent to the substrate is mainly based on Al2O3xe2x80x94SiO2, Al2O3xe2x80x94MgO, Al2O3xe2x80x94ZrO2 or Al2O3; and the spray coating layer adjacent to the material to be fired is mainly based on ZrO2, Al2O3 or Al2O3xe2x80x94MgO. An Al2O3 content is preferably 70 to 95 wt % for the substrate, 98 wt % or more for the spray coating layer adjacent to the substrate and 72 to 96 wt % for the total in the substrate and the spray coating layer adjacent to the substrate.
An SiO2 content is preferably 0.5 wt % or less for the spray coating layer adjacent to the substrate and 0.05 to 0.5 wt % for the spray coating layer adjacent to the material to be fired.
In a firing jig for an electronic element according to this invention, a porosity in each of the spray coating layers is preferably 16% or less. The spray coating layers preferably have a crack penetrating each of these layers with a width of 1 to 5 xcexcm. The total thickness of the spray coating layers is preferably 50 to 1000 xcexcm. A particle size in each of the spray coating layers is preferably 300 xcexcm or less.
This invention also provides a firing jig for an electronic element comprising a substrate made of a ceramic and a spray coating layer, wherein a surface adjacent to the substrate is the spray coating layer and the surface of the spray coating layer is a flame coating layer.
A firing jig for an electronic element according to this invention is based on an alumina-silica material, in which performance of the electronic element to be placed and fired on the firing jig may be stabilized by forming two or more spray coating layers on the surface of a substrate and defining the contents of the components required for the substrate and the individual spray coating layers to minimize the amount of SiO2 entering into the coating layer. This invention will be described with reference to, but of course not limited to, embodiments below.
The term, xe2x80x9ca firing jig for an electronic elementxe2x80x9d as used herein means a member used for firing of an electronic element made of a ceramic such as a ceramic capacitor, a thermistor and ferrite, which may comprise, for example, a setter, a case, and a plate for placing a material to be fired.
A firing jig of this invention comprises two or more spray coating layers on the surface of a substrate, where these layers are made of different materials. Thus, the surfaces adjacent to the surface of the substrate and to the material to be fired are spray coating layers with a smaller porosity than that in a flame coating layer to prevent a component adversely affecting a work property or deteriorating the substrate from transferring. Furthermore, if forming a flame coating layer, it may be formed by coating with melted ceramic particles, and thus the layer is a solid hard layer so that expansion or shrinkage cannot be absorbed in the layer, causing detachment. On the other hand, in a spray coating layer, particles are mutually sintered to form a soft layer, allowing intralayer relaxation. When a spray coating layer is formed on the surface of the substrate, the layer can prevent a reactive component in the substrate from being in contact with the material of the electronic element, allowing a silica-alumina material including silica adversely affecting the properties of the electronic element to be used as a substrate.
The term xe2x80x9cspray coatingxe2x80x9d as used herein refers to a process of preparing a ceramic powder slurry for spraying or applying the slurry on the surface of the substrate before firing. The term xe2x80x9cwork propertyxe2x80x9d refers to dispersion of a dielectric constant generated in an electronic element when firing the electronic element using the firing jig. A good work property in a firing jig means that dispersion property of a dielectric constant is good in an electronic element fired using the firing jig.
The spray coating layers are preferably mainly based on Al2O3xe2x80x94SiO2, Al2O3xe2x80x94SiO2xe2x80x94MgO or SiC. It is preferable that the spray coating layer adjacent to the substrate is mainly based on Al2O3xe2x80x94SiO2, Al2O3xe2x80x94MgO, Al2O3xe2x80x94ZrO2 or Al2O3 while the spray coating layer adjacent to the material to be fired is mainly based on ZrO2, Al2O3 or Al2O3xe2x80x94MgO. Such a configuration may provide a firing jig comprising a substrate with good spall resistance and vent resistance, an intermediate layer preventing a component adversely affecting a material to be fired from transferring and the outer layer less reactive to the material to be fired.
In this invention, an Al2O3 content is preferably 70 to 95 wt % for the substrate. If the Al2O3 content of the substrate is less than 70%, a thermal conductivity of the substrate is so reduced that a temperature in the firing jig may significantly vary, leading to an uneven work property on the jig. If the Al2O3 content of the substrate is more than 95%, a thermal expansion coefficient of the substrate is so increased that a stress due to temperature variation may be significantly increased, leading to crack formation. An Al2O3 content is preferably 98 wt % or more for the spray coating layer adjacent to the substrate, and the total Al2O3 content of the substrate and the spray coating layer adjacent to the substrate is preferably 72 to 96 wt %. Furthermore, as described later with reference to Examples, an alumina content of less than 70 wt % in the substrate may improve heat resistance, but relatively increased silica component may adversely affect the properties of an electronic element as a material to be fired. The other alumina contents may be limited within the above range to stabilize the properties of the electronic element.
In the present invention, an SiO2 content is preferably 0.5 wt % or less for the spray coating layer adjacent to the substrate, and 0.05 to 0.5 wt % for the spray coating layer adjacent to the material to be fired. Thus, dispersion of a component adversely affecting a material to be fired by passing through the layer adjacent to the substrate as an intermediate layer can be prevented. Furthermore, the SiO2 content in the layer adjacent to the material to be fired as the outer layer may be limited within the above range so that absorption of SiO2 contained in the material to be fired as a firing aid into the firing jig may be prevented.
In particular, the spray coating layer adjacent to the material to be fired must be made of a material less reactive to the material for the electronic element, but may depend on the type of the electronic element. For example, since the ceramic condenser is made of barium titanate, zirconia less reactive to the material is preferably selected. In this invention, the main component for the coating layer is preferably one or more selected from the group consisting of non-stabilized ZrO2, Y2O3 stabilized ZrO2, Y2O3 partially stabilized ZrO2, CaO stabilized ZrO2 and CaO partially stabilized ZrO2.
In this invention, a porosity in each of the spray coating layers is preferably 16% or less. Thus, penetration of a component which may deteriorate a substrate by reacting with the substrate after being dispersed from the work may be blocked by the spray coating layers, and dispersion of SiO2 from the substrate to the outer surface may be prevented in firing of a ceramic capacitor, resulting in improved stability in an electronic element.
The spray coating layers preferably have a crack penetrating each of these layers with a width of 1 to 5 xcexcm. In firing of a ceramic capacitor or ferrite, whose outer surface is often made of zirconia, as the number of passing through a kiln increases, residual expansion characteristic in zirconia may be generated. Since it may cause curving or detachment in a firing jig, it is effective that a crack as defined above exists as a room for absorbing such residual expansion.
The total thickness of the spray coating layers is preferably 50 to 1000 xcexcm. Furthermore, as described later in Examples, if the thickness is less than 50 xcexcm, properties of an electronic element fired on the firing jig may vary, while if it is more than 1000 xcexcm, detachment between layers in the firing jig may be often caused when firing an electronic element using the firing jig. The layer adjacent to the substrate must firmly adhere to the substrate. It is preferable that the surface of a lower layer is coarser for firm adhesion of the lower layer to the layer adjacent to the material to be fired, and coarse particles may be appropriately added to the layer adjacent to the substrate as an intermediate layer to make the surface rough, which is suitable to stably hold a layer deposited on the surface layer. However, if an excessive amount of coarse particles or excessively large particles are used, an adhesive strength in the intermediate layer may be reduced, causing detachment. Furthermore, when the layer adjacent to the material to be fired as the outer layer is spray-coated, the use of excessively large particles is not preferable because it may reduce an adhesive strength with the lower layer and detachment may be caused as described above. A particle size in each of the spray coating layers is preferably 300 xcexcM or less.
In this invention, an intermediate layer is baked during spray coating and thus blasting of a substrate or preheating before forming the intermediate layer may be not necessarily required for preventing detachment of the intermediate layer from the substrate.
In a firing jig for an electronic element comprising a ceramic substrate of this invention, the surface adjacent to the substrate may be a spray coating layer, whose surface may be a flame coating layer. In this jig, the surface layer is a flame coating layer while the surface adjacent to the substrate is a spray coating layer. Thus, thermal expansion or shrinkage of the substrate and the flame coating layer may be absorbed. It may prevent deformation of the basedue to detachment of the flame coating layer or residual expansion in the flame coating layer.
The term xe2x80x9cflame coatingxe2x80x9d as used herein refers to a process for forming a flame coating film by melting metal or ceramic fine powder (hereinafter, referred to as xe2x80x9cflame coating materialxe2x80x9d) by heating and then spraying it onto an object. There may be variations such as gas flame coating using flame and arc flame coating using arc depending on a heating method. In this invention, it is preferable to form an outer flame coating film by plasma flame coating using a plasma jet.
In this invention, particularly preferable plasma flame coating is water-stabilized plasma flame coating. The minimum film thickness of a flame coating film by gas plasma flame coating is about 20 to 50 xcexcm because a thick coating film with the minimum film thickness of about 100 xcexcm may be formed by water-stabilized plasma flame coating. Since water-stabilized plasma flame coating may form a coating film with a relatively porous and coarse surface, it is also preferable in terms of improvement of adhesiveness to the surface of the spray coating layer adjacent to the substrate.
This invention will be more specifically described with reference to, but not limited to, Examples.